In the field of toners for electrophotography, with the progress of electrophotographic systems, it has been demanded to develop toners adaptable for high image quality and high copying speed. From the viewpoint of the high image quality, it has been required that the toners are in the form of finely divided particles and have various properties such as a good heat-resistant storage property and a wide fusing temperature range.
There are conventionally known methods in which the toners are improved in heat-resistant storage property by using a resin having a high glass transition point therein. However, in these methods, the resulting toners tend to be deteriorated in low-temperature fusing ability. Also, in order to ensure a broad fusing temperature range of the toners, it may be effective to increase a content of high-molecular weight components in a resin for the toners. However, in the case of bulk polymerization, it will be difficult to pulverize the resulting resin while inhibiting reduction in a molecular weight of the high-molecular weight components in the resin. In addition, in the method for producing a toner by polycondensation in an aqueous medium, it is substantially difficult to produce a high-molecular weight compound. Further, in the case of an emulsification and aggregation method, etc., there tend to occur problems such as need of a large amount of a solvent or a large mechanical force and occurrence of decomposition of the high-molecular weight compound when emulsified.
There are disclosed a method for enhancing a fusing ability of a toner in which a carbodiimide compound is added to emulsified particles obtained by polycondensation in water to form a chemical bond therebetween on a surface of the respective emulsified particles (Patent Document 1), and a dry toner constituted of toner particles which are obtained by melt-kneading a resin binder having a carboxyl group or an acid anhydride group, a colorant, a specific amount of an oxazoline-based compound having two or more oxazoline groups in a molecule thereof or a resin component containing an oxazoline group, and a releasing agent (Patent Document 2). Also, there is disclosed a toner obtained by a method in which raw materials of the toner including a polyester-containing resin binder and a releasing agent are melt-kneaded together, and then the resulting melt-kneaded material is emulsified in an aqueous medium (Patent Document 3). On the other hand, there has been developed a toner whose surface is treated with an external additive containing two kinds of inorganic particles which are different in particle size from each other in order to enhance a durability and a storage property thereof (Patent Document 4).
Patent Document 1: JP 2006-317715A
Patent Document 2: JP 2000-292968A
Patent Document 3: JP 2007-279195A
Patent Document 4: JP 8-227171A